THE MECHANISM OF INACTIVATION OF HUMAN PLACENTAL S-ADENOSYLHOMOCYSTEINE HYDROLASE BY (E)-4',5'DIDEHYDRO-5'-METHOXYADENOSINE AND ADENOSINE 5'-CARBOXALDEHYDE OXIME

The mechanisms by which (E)-4',5'-didehydro-5'-methoxyadenosine (DMOA) and adenosine 5'-carboxaldehyde oxime (ACAO) inactivate S-adenosylhom ocysteine (AdoHcy) hydrolase were elucidated in this study. Their inhi bitory activities toward AdoHcy hydrolase were found to be time- and c oncentration-dependent, and DMOA and ACAO had K-1 and k(2) values of 3 .0 mu M and 0.10 min(-1) and 0.67 mu M and 0.16 min(-1), respectively. The inactivation of AdoHcy hydrolase by DMOA (and ACAO) occurs concom itantly with the reduction of the enzyme-bound NAD(+) to NADH. The rat es of enzyme inactivation correspond to the rates of NADH formation. I ncubation of both DMOA and ACAO with the NAD(+) form of AdoHcy hydrola se resulted in formation of 3'-ketoadenosine (3'-keto-Ado) 5'-carboxal dehyde and its 4'-epimer. Incubation of DMOA and ACAO with the apo for m of the enzyme afforded adenosine (Ado) 5'-carboxaldehyde and its 4'- epimer. These results show that DMOA and ACAO are ''proinhibitors'' of the enzyme. They are first converted to the inhibitors (Ado 5'-carbox aldehyde and its 4'-epimer) in the active site of the enzyme; these in hibitors then inactivate the enzyme by a type I mechanism. The results from this study demonstrated that this is a common mechanism by which 4',5'-didehydroadenosine analogs, serving as substrates of both the 5 '-hydrolytic activity and the 3'-oxidative activity of the enzyme, ina ctivate AdoHcy hydrolase, The results also provide further evidence su pporting the hypothesis that AdoHcy hydrolase possesses a 5'-hydrolyti c activity independent of the 3'-oxidation activity. (C) l997 Academic Press